Unraveling the effect of aromaticity for the dynamics of excited states of single benzene fluorophores

Abstract

The photophysical properties of a series of recently synthesized single benzene fluorophores were investigated using ensemble density functional theory calculations. The energetic stability of the ground and excited state species were counterposed against the aromaticity index derived from local vibrational modes. It was found that the large Stokes shift of the fluorophores (up to ca. 5800 cm ${}^{-1}$ ) originates from the effect of electron donating and electron withdrawing substituents rather than $\pi$ -delocalization and related (anti-)aromaticity. On the basis of nonadiabatic molecular dynamics simulations, the absence of fluorescence from one of the regioisomers was explained by the occurrence of easily accessible S ${}_1$ /S ${}_0$ conical intersections below the vertical excitation energy level. It is demonstrated in the manuscript that the analysis of local mode force constants and the related aromaticity index represent a useful tool for the characterization of $\pi$ -delocalization effects in $\pi$ -conjugated compounds.

Keywords: aromaticity; ensemble density functional theory; excited state; local vibrational mode theory; single benzene fluorophores.